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comparison to right hand side. The analysis of hypsometric curve reveals that the
potential dissection is more at Tindharia and for the stretch along the Hill Cart Road
between Gayabari to Mahanadi including Paglajhora. The northeastern part of the
basin near the water divide is extensively dissected and shows absolute instability.
The study area is composed mainly of the Darjiling Gneiss, Daling formation
composed of Chungtung formation, Lingtse Granite, Garubathan formation and
Ryang formation. Gondwana formation, the most fragile one due to the presence of
carboniferous rocks is located along a narrow belt being sandwiched between
Daling to the north and Siwalik to the south. The structural-cum stratigraphic
succession can be observed as a traverse across Tindharia-Kurseong region. The
concerned study area is structurally instable as most of the unconformities are lying
across the drainage lines and so subsidence zones are developed at the junctions of
the drainage lines with the structural discontinuities and lineaments. The Paglajh-
ora, the biggest subsidence zone of the study area, is situated along the Darjiling-
Daling boundary. The mica schist in the Daling series is also a factor of instability
as most of the slides are located on the mica schist due to its less resistance. The
slope is maximum near Paglajhora area and the Hill Cart Road and North Eastern
Frontier Rail line (0.61 m gauge) cross the entire river system twice through this
steeper and unstable zone. The slope is least at the central part where the river
develops a cut and
fill terrace. The study shows that the LPI for the slope categories
are increasing at a steady rate as the steepness increases and it is the indicator of the
direct control of slope on the slope failure. All the cells having steepness of 19
°
and above are affected by landslide. The positive curvature is more common
indicating the tendency of immediate drainage of surface water which is detrimental
to the stability of both soil and slope. Paglajhora, Gayabari, Tindharia, and Shiviter
are dominated by the moderate to high levels of positive and negative surface
curvature with moderate levels of slope surface dissection. Landslide prone north,
south, east, north east and south east facets are closely associated with maximum
slope and relief which is found at upper and lower Paglajhora, Shiviter T.E.,
Gayabari Lower slope and Tindharia where landslide potentiality is high. Sub-
surface soil over the steep slope at the places of Tindharia T.E. and Lower Pag-
lajhora is dominated by humus which is very loose, crumble and friable. Such
humus dominated soil gets saturated very easily due to moderate amount of rain and
reduces cohesion by increasing pore-water pressure and make the slope surface
more vulnerable to soil slip. At marginal parts of the basin basically on the both
sides of the Hill Cart Road from Tindharia to Gayabari, upslope parts from Pag-
lajhora proper, T.E. and Shiviter upslope where the saturated soil depth is less than
1.75 m. At all these places sub-surface soil gets saturated quickly and promotes
suitable condition for shallow soil slip. At the sub-surface layer of the soil per-
centage of pore space is high but at greater depth pore space decreases because of
the existence of large percentage of
°
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23
finer particles. The reduction of pore space at
greater depth results in the increase of water holding capacity and volumetric
expansion at the sub-surface soil which increases the pore-water pressure and
reduces cohesion and
finally invites slope soil failure at most of the places of the
Shivkhola Watershed.
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